PROJECT SUMMARY/ABSTRACT Candida albicans is the leading pathogenic fungus of humans as well as a ubiquitous constituent of the microbiome, occupying the gastrointestinal (GI) tract, in addition to the skin, oral cavity and the genitourinary tract. Commensal yeast may transition to an infectious organism in susceptible patients, manifesting in skin and soft tissue cellulitis, oropharyngeal/esophageal candidiasis and septicemia. Antibiotic-driven microbial dysbiosis, immunocompromised HIV and diabetic individuals, and those receiving surgery or indwelling devices are particularly at risk for acquiring candidiasis, resulting in significant clinical complications for which antifungal treatment is the first line of defense. In addition to diverse engagement of the host, C. albicans exhibits a remarkable morphologic plasticity in response to environmental cues, adopting six distinct yeast-like cell shapes with specific transcriptional responses. These observations suggest that this fungus is particularly well adapted to thrive in multiple host environmental niches and potentially evade antifungal treatment in part by utilizing cell type changes. The specific environments in which morphologic plasticity allows for fungal establishment, however remain largely unknown. Additionally, while many studies have documented the ability of white-phase yeast to incite disease, a paucity of data exists defining specific fungal effectors mediating fitness in the host. The long-term goal of this project is to understand the mechanisms by which C. albicans succeeds as a commensal and pathogen in an array of host environments. To uncover the molecular underpinnings of these complex fungal-host interactions, established mouse models that mimic human colonization and disease states will be utilized in two aims. Aim 1 will be directed at understanding the role of colony morphology in C. albicans niche establishment with the host. The six yeast-like cell types will be used in 5 distinct infection models: gastrointestinal and skin commensalism, skin cellulitis, oropharyngeal/espophogeal candidiasis and systemic disease. Mice will be analyzed for fungal burden and cytokine response profile within host tissues in addition to visualization of cell morphology maintenance at the host-fungal interface through scanning electron microscopy and histological imaging. These studies will provide comparative and comprehensive understanding of fungal morphotypes specialized for host niches. In Aim 2, high-throughput forward genetic screens will be performed by infecting this skin of mice in both commensal and cellulitis models with pools of barcoded mutants from a comprehensive C. albicans deletion library. Challenge with yeast in both skin models will reveal previously undescribed genetic regulators and effectors contributing to fitness in both colonization and pathogenesis. In aggregate these innovative research strategies will garner a greater knowledge of the mechanisms of fungal infection and provide new avenues for the evaluation and treatment of candidiasis.